Linear data storage media such as magnetic tape provides a means for physically storing data which may be archived or which may be stored in storage shelves of automated data storage libraries and accessed when required. One method for maximizing the amount of data that can be stored is to maximize the number of parallel tracks on the media, and this is typically accomplished by employing servo systems which provide track following and allow the tracks to be spaced very closely.
An example of track following servoing is the provision of prerecorded parallel longitudinal servo tracks that lie between groups of longitudinal data tracks, so that one or more servo heads may read the servo tracks and an accompanying track following servo will adjust the lateral position of the head or the tape to maintain the servo heads at a desired lateral position with respect to the servo tracks such that the data heads are centered with respect to the data tracks.
An example of a track following servo system comprises a timing based servo system of the incorporated U.S. Pat. No. 5,689,384. A timing based servo system is employed, for example, with the Linear Tape Open (LTO) format, one example comprising the IBM LTO Ultrium magnetic tape drive and associated tape cartridge. A linear servo track comprises a sensible transition pattern, for example, of prerecorded magnetic transitions forming a timing based servo pattern of a repeating cyclic periodic sequence of transitions of two different azimuthal orientations that extend laterally over the linear servo track. For example, the pattern may comprise transitions slanted, or having an azimuthal orientation, in a first direction with respect to the direction of the linear servo track, alternating with transitions slanted, or having an azimuthal orientation, in the opposite direction. Thus, as the medium is moved with respect to a servo read head in the linear direction, the lateral positioning of the servo read head with respect to the timing based servo track is sensed based on a measure of time between two transitions having different azimuthal orientation as compared to time between two transitions having parallel azimuthal orientation. The relative timing of the transitions read by the servo read head varies linearly depending on the lateral position of the head. Thus, a number of parallel data tracks may be aligned with different lateral positions across the servo track.
Synchronization of the servo read head and decoder to the servo pattern may be accomplished by having two separate sets of transitions, each set comprising a grouping of a different number of pairs of transitions, one set comprising a grouping having, for example, 4 pairs of transitions, and another set comprising a grouping having 5 pairs of transitions. Thus, the lateral position of a servo read head with respect to the servo track may comprise a measure of time between two transitions having different azimuthal orientation, e.g., between the first transition of a pair in one set and the other transition of the pair, this distance called the “A” distance; as compared to time between two transitions having parallel azimuthal orientation, e.g., between the first transition of a pair in one set and a similar first transition of another pair in another set, called the “B” distance.
The prior linear servo track timing based servo pattern is generated by a servo writer having two spaced apart write elements of different azimuthal orientations, forming the “A” distance. A drive moves the linear data storage medium across the write elements at a predetermined velocity, and a source of timed pulses causes the write elements to write a single pair of transitions for each pulse, such that the pattern of pairs of transitions are written on the linear data storage medium.
In theory, the format is extendable to higher track pitches, wherein the data tracks are closer together. The “A” geometric distance is determined photolithographically, and is independent of the timing of the pulses or of the velocity of the servo writer drive.
However, with the prior servo writer generator utilizing two spaced apart elements with different azimuthal orientations, the writer generator is pulsed periodically with the period between pulses set so that, with the nominal tape velocity of the servo pattern writer, the geometric distance between patterns is the “B” distance mentioned above. Thus any error in the velocity of the tape in the servo writer results in an error in the “B” distance and hence an error in the lateral position calculated based on pulse “B” pulse timing assuming the correct “B” distance. Hence, the precision of the “B” geometric distance between the first transition of a pair in one set and a similar first transition of another pair in another set, is dependent upon the precision of the velocity of the tape in the servo writer drive and the precision of the timing between the pulses, so that the similar first transition of another pair in another set of transitions may be misregistered with respect to the first transition of the pair in the one set. Thus, with a given pulse timing, the distance between transitions determining the “B” distance is strictly proportional to the velocity of the tape in the servo writer. Servo writer velocity error introduces a servo position error to the servo track following system and results in data track misregistration.
Further, the data track misregistration becomes worse for data tracks that are positioned such that the distance between “A” pulses is closer to the distance between “B” pulses.